Off-axis return beam tube

ABSTRACT

An electron beam tube of the return beam type in which the return beam follows a distinct path from the path of the primary beam to permit placement of a detector separated from the source at a position of minimum return beam diameter. A magnetic deflection field is provided to steer the primary beam from an electron gun on-axis into the focusing and deflection system for scanning of a target surface, and to oppositely steer the return beam off-axis for detection.

[22] Filed:

United States Patent 1 Rutherford, Jr.

[ 1 May 29, 1973 V [54] OFF-AXIS RETURN BEAM TUBE [75] inventor:

Robert E. Rutherford, Jr., New Canaan, Conn.

[73] Assignee: Columbia Broadcasting System, Inc.,

New York, N.Y.

June 28, 1971 [21] Appl. N0.: 157,143

[52] US. Cl. "315/11, 313/67, 315/12,

315/31 R [51] Int. Cl ..H0lj 31/48 [58) Field oiSearch ..315/10,11, 12,31 R;

[56] References Cited UNITED STATES PATENTS 3,072,819 1/1963 Stemglass ..3l5/11 3,469,141 9/1969 Hall et al. ..315/1O 3,461,332 8/1969 Sheldon ..3l3/65 R 2,887,595 5/1969 Rijssel et al.... ...315/11 X 2,863,087 12/1958 Barbier ..3l5/l1 Primary Examiner-Carl D. Quarforth Assistant Examiner-4. A. Nelson Attorney-Spencer E. Olson [57] ABSTRACT An electron beam tube of the return beam type in which the return beam follows a distinct path from the path of the primary beam to permit placement ofa detector separated from the source at a position of minimum return beam diameter. A magnetic deflection field is provided to steer the primary beam from an electron gun on-axis into the focusing and deflection system for scanning of a target surface, and to oppositely steer the return beam off-axis for detection.

10 Claims, 5 Drawing Figures Patented May 29, 1973 3,736,460

2 Sheets-Sheet 1 INVENTOR ROBERT E. RUTHERFORQJF ATTORNEY Patented May 29, 1973 3,736,460

2 Sheets-Sheet 2 INVENTOR ROBERT E. RUTHERFORD, JR.

BY 4, a424 ATTORNEY OFF-AXIS RETURN BEAM TUBE FIELD OF THE INVENTION This invention relates to electron beam tubes and more particularly to return beam tubes in which the returned electron beam is diverted and detectedalong an axis different from the axis of the primary beam.

BACKGROUND OF THE INVENTION In an electron beam tube of the return beam type, such as an image orthicon camera tube, the primary beam from an electron gun is directed along the longitudinal tube axis through a focusing and deflection sys tem for scanning of a target surface, and a beam returned from the target surface is caused to follow substantially the same path for detection at a point near the electron source. A positive charge distribution formed on the target surface, which in the case of a camera tube corresponds to the brightness pattern of a viewed scene, is scanned by the primary beam which is altered in density in accordance with the charge pattern. The return beam is thus amplitude modulated in response to the charge pattern, and is detected to provide a video output signal representative of this pattern being scanned.

In a conventional return beam tube, the return beam detector, typically an electron multiplier, is disposed closely adjacent and usually surrounding the electron gun to intercept the returned beam. The position of the detection surface is necessarily different from the origin of electrons at the electron gun cathode and this difference between source and detector positions prevents precise focusing of the returned beam. Moreover, the type of electron detector employed in an electron beam tube of conventional construction is limited by the required placement of the detector closely adjacent the electron gun. While on-axis beam tube structures operate with sufficient accuracy for many purposes, in order to satisfy higher performance criteria sometimes required, it would be desirable to provide a more precisely focused return beam or a beam of minimum diameter.

SUMMARY OF THE INVENTION In accordance with the present invention, an electron beam tube is provided in which the return path is diverted from the incident path such that the return beam can be received and detected at a plane of minimum return beam diameter, and a preferred detector employed without physical interference with the electron gun source. In brief, both the electron gun and electron detector are disposed off-axis from the longitudinal axis of the tube, and a magnetic field provided for diversion of the primary beam from the electron gun on-axis for focusing and deflection onto a target surface, and for opposite diversion of the return beam off-axis for detection at a position separated from the source and at a plane of minimum beam diameter. Typically, the magnetic field is provided across the gap of a toroidal magnet located adjacent the focusing electrode, with the magnetic field oriented with respect to the angular paths of the primary and return beams and with respect to the tube axis to provide intended opposite steering of the respective primary and return beams. The invention is also useful with charged particles other than electrons and can be employed with plural beam sources and detectors angularly separated with respect to the tube axis.

DESCRIPTION OF THE DRAWINGS The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a cutaway pictorial view of an electron beam tube embodying the invention;

FIGS. 2 and 3 are schematic diagrams of electron beam trajectories in a magnetic field useful in illustrating operation of the invention;

FIG. 4 is a schematic representation of an alternative embodiment of the invention; and

FIG. 5 is a schematic representation of a further embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION An electron beam tube of the return beam type constructed and operative according to the invention is depicted in FIG. 1 and includes an electron gun It) operative to provide a well defined beam of electrons along an axis 12 which is disposed at an angle 0 with respect to the longitudinal tube axis 14. A return beam detector 16, typically a semiconductor electron multiplier, is disposed with the detecting surface 18 thereof in a plane of minimum diameter of the return beam 20 which is angula'rly disposed with respect to axis 14 at the angle 0 but on the opposite side of axis 14 as the incident path 12. An electromagnet 22, comprising a to mid 24 of magnetizable material having an energizing coil 26 therearound and an air gap 28, is disposed as illustrated with the gap 28 oriented with respect to paths 12 and 20 and longitudinal axis 14 to provide a magnetic field thereacross orthogonal to axis 14 to achieve beam diversion according to the invention.

symmetrically disposed around the tube axis 14 is a focusing electrode 30 in the form of a conductive cylinder, and an electrostatic deflection system 32 which includes a first pair of confronting plates 34 and 36 and an orthogonally arranged pair of confronting plates 38 and 40, each pair of plates providing deflection along a respective orthogonal axis. A target plate 42 having a target surface 44 is disposed as illustrated for scanning of the target surface by the deflected beam. A mesh electrode 46 is supported by a mounting ring 48 adjacent target surface 44 and is operative to divert the deflected electron beam 50 into a path normally incident upon the target surface 44 and to decelerate the electrons to zero or near-zero landing energy at the target surface, as is desirably required in a return beam tube for electron reflection from surface 44.

The structure described above, with the exception of the novel off-axis disposition of source It) and detector 18 and the magnetic field for diversion of the primary and return beams, is operative in a well known manner. A suitable envelope (not shown) is provided around the tube elements to provide an evacuated enclosure, and suitable energizing potentials are applied to the electrodes for intended tube operation. Typically the tube structure is employed in an image orthicon camera tube which additionally includes a photocathode onto which a viewed scene is imaged, and which emits a pattern of photoelectrons onto the surface of target 42 opposite surface 44 to provide a charge pattern representative of the optical image. The primary beam scanned across target surface 44 by action of the suitably energized electrodes 34, 36, 38 and 40 is altered, as it approaches the target surface, in accordance with the charge distribution on this surface. The beam returned from surface 44 has an electron density which varies correspondingly with the charge distribution, the variable density beam being converted by detector 18 to an amplitude varying output signal representative of the scanned charge pattern.

By virtue of the invention, the primary and return beams traverse separate paths to permit physical displacement of electron gun l and detector 18 and to achieve detection of the return beam at a plane of minimum beam diameter, which can be at the focal plane of the return beam. Diversion of the primary and return beams is illustrated diagrammatically in FIGS. 2 and 3, in both of which figures a magnetic field B is depicted extending in a plane orthogonal to the plane of the page '(into the page). In FIG. 2, there is shown a primary beam entering the magnetic field B along a plane orthogonal to the plane of field B and along path 12 in this orthogonal plane which is angularly oriented with respect to tube axis 14 at an angle 6. This beam entering the magnetic field along path 12 experiences a force equal to the cross product of the electron velocity and field strength, to cause exit of the beam from the field along axis 14, the beam continuing through the focusing and deflection electrodes 30 and 32 and mesh electrode 46 for scanning of surface 44.

The return beam, as illustrated in FIG. 3, after reflection back through the deflection and focusing electrodes, enters the magnetic field along tube axis 14 in a direction opposite to that of the primary beam and experiences a force of opposite polarity to cause diversion of the beam along angular path 20 which is of equal but opposite orientation with respect to axis 14. The return beam continues along path 20 for detection by detector 18, located at the detection plane to which the beam can be accurately returned as a result of the distinct off-axis path provided by the invention.

From the foregoing, it should be evident that the offaxis disposition of electron gun It) and detector 18 affords considerable improvement in performance over on-axis tube structures by reason of the highly precise focusing of the return beam achieved. The invention also permits use of detectors which would otherwise be physically unsuitable for use in conventional on-axis tube structures by reason of the physical constraints of such structures. The exact angular orientation of the primary and return beams is not especially critical but can be readily determined in relation to the desired magnetic field strength in a particular embodiment for achieving intended beam diversion. In a typical implementation of the invention, an angular disposition of 15 has been employed with a field strength of 50 gauss across a V: inch gap 28. Appropriate magnetic shielding can be provided in a well known manner to prevent interference by the diversion field with operation of the accompanying tube structure.

In the embodiment of FIG. ll, the return beam has an energy level essentially the same as that of the primary beam and thus experiences a substantially equal but opposite deflection force during passage through the magnetic field across gap 28. In this instance, therefore, the source and detector 116 are disposed at equal and opposite angles with respect to tube axis 14. When, however, the energy of the return beam is different from that of the primary beam, the angular disposition of the electron gun and detector will not be the same with respect to the tube axis, but will be at respective angular dispositions dictated by the respective energy levels of the electron beams in conjunction with the field strength of the deflection field across gap 28. It should also be noted that the magnetic field across gap 28 need not be uniform, but can be of many well known configurations to provide intended beam diversion, the exact placement of the electron gun and return beam detector being determined in accordance with the particular magnetic field employed.

An alternative embodiment of the invention is illustrated in FIG. 4 wherein more than one electron beam source is provided within the same tube envelope. An electron gun 50 is angularly disposed with respect to the tube axis 52 at an angle 0,, while a second electron gun 54 is disposed at an angle 0 -with respect to'axis 52' and in the same plane as gun 50. A detector 56 is disposed on the opposite side of axis 52 at an angle 6 therewith and also in the same plane as the electron guns. Each source 50 and 54 is operative to direct a respective electron beam along respective paths 58 and 60 into a magnetic field provided across gap 64 to cause diversion of the respective beams onto the tube axis 52. The energy levels of each electron beam are determined in conjunction with the respective angular dispositions 49 and 0 to accomplish the requisite beam diversion onto axis 52 for subsequent focusing and deflection onto a target surface, as described above. The return beam entering the magnetic field along axis 52 in a direction opposite to the primary beam is oppositely diverted, as described above, into a path 62 for receipt by detector 56. In the illustrated embodiment of FIG. 4, the return beam corresponding to each primary beam from guns 50 and 54 is diverted along a common path 62 for detection. Separate detectors can also be provided to receive respective return beams in those instances where the respective return beams experience different degrees of diversion.

A further embodiment of the invention is depicted in FIG. 5 wherein a second electron source and associated detector are angularly arranged with respect to a first source and detector pair. An electron gun 66 is angu larly disposed on one side of a tube axis 82, with an associated detector 68 disposed in opposite angular disposition with respect to the tube axis. A second electron gun 70 and detector 72 are similarly disposed with respect to axis 82 but in a plane orthogonal to the plane of gun 66 and detector 68. A magnetic field is provided across gap 84 and can be oriented by physical rotation of the gap (as shown in dotted outline) to provide intended beam diversion for each gun and detector pair. Alternatively the magnetic field can be electrically rotated by use of appropriate field producing means to achieve intended beam diversion for each gun and detector pair. Gun 66 is operative to direct an electron beam along path 74 into the magnetic field provided by gap 84 for diversion of this beam along axis 82 for focusing and deflection onto a target surface. The return beam from the target surface is oppositely deflected along path 76 for receipt by detector 68. Similarly, gun 70 is operative to direct an electron beam along path 78 for diversion by the magnetic field, which is oriented orthogonally to the field for gun 66, onto the tube axis 82, the return beam being oppositely diverted along path 80 for detection at the detector 72.

It will be appreciated that the invention can be employed in various return beam tube configurations such as in camera tubes, discussed hereinabove, and scanning electron microscopes, and that various modifications and alternative implementations will occur to those versed in the art. For example, an electron beam tube according to the invention can have other focusing and deflection systems of either electrostatic or magnetic types rather than the specific electrostatic system described above. Moreover, the invention can be employed with charged particles other than electrons. Accordingly, it is not intended to limit the invention by what has been particularly shown and described except as indicated in the appended claims.

What is claimed is:

1. An electron beam tube comprising:

an elongated envelope having a longitudinal axis;

a target surface supported near one end of said envelope for receiving an electron beam scanned thereacross;

focusing means supported within said envelope and disposed symmetrically around said longitudinal axis for focusing an electron beam onto said target surface;

deflection means supported within said envelope between said focusing means and said target for scanning said electron beam across said target surface;

an electron gun supported within said envelope near the other end thereof and disposed off-axis from said longitudinal axis and operative to direct an electron beam along a first path angularly incident with said longitudinal axis at a point thereon intermediate said electron gun and said deflection means;

an electron detector supported within said envelope near said other end thereof and disposed off-axis from said longitudinal axis at an equal and opposite angle from said electron gun and operative to receive an electron beam returned from said target surface along a second path of opposite angular disposition with respect to said tube axis as said first path; and

means for providing a magnetic field across said longitudinal axis and oriented with respect to said longitudinal axis and said first and second angular paths to divert said electron beam at said point from said first angular path onto said longitudinal axis and to oppositely divert said returned electron beam at said point from said longitudinal axis onto said second angular path.

2. An electron beam tube according to claim 1 wherein said means for providing a magnetic field is operative in conjunction with the energy levels of said electron beam and said returned electron beam to produce equal and opposite angular diversion of said electron beam and said returned electron beam.

3. An electron beam tube according to claim 1 wherein said first and second paths are in equal angular disposition on respective opposite sides of said longitudinal axis; and

said means for providing a magnetic field is operative to produce equal and opposite angular diversion of the respective electron beams directed along said respective first and second paths.

4. An electron beam tube according to claim 1 wherein said magnetic field is orthogonal to said longitudinal axis and said first and second paths are in a plane orthogonal to said magnetic field.

5. An electron beam tube according to claim 4 wherein said magnetic field providing means includes an electromagnet having a gap oriented with respect to said longitudinal axis and said first and second paths and across which said magnetic field is produced.

6. An electron beam tube according to claim 5 wherein said electromagnet includes a toroidal core having an energizing coil therearound.

7. An electron beam tube according to claim 1 including:

a second electron gun supported within said envelope near said other end thereof and disposed off-axis from said longitudinal axis and operative to direct a respective electron beam along a third path angularly incident with said longitudinal axis at said point and in a plane angularly rotated from the plane of said first path; and

a second electron detector supported within said envelope near said other end thereof and disposed off-axis from said longitudinal axis on a side opposite from said second electron gun and operative to receive a respective electron beam returned from said target surface along a fourth path of opposite angular disposition with respect to said longitudinal axis as said third path and in the same plane thereof;

said means for providing a magnetic field being also operative to divert said electron beam from said third path at said point onto said longitudinal axis and to oppositely divert said respective returned electron beam at said point from said longitudinal axis onto said fourth path.

8. An electron beam tube comprising:

an elongated envelope having a longitudinal axis;

a target surface supported within said envelope near one end thereof for receiving an electron beam scanned thereacross;

focusing means supported within said envelope and disposed symmetrically around said longitudinal axis for focusing an electron beam onto said target surface;

deflection means supported within said envelope between said focusing means and said target for scanning said electron beam across said target surface;

a plurality of electron guns supported within said envelope near the other end thereof each disposed off-axis from said longitudinal axis by a different angle and each operative to direct a respective electron beam of different energy level along a respective first path angularly incident with said longitudinal axis at a point thereon intermediate said electron guns and said deflection means;

an electron detector supported within said envelope near said other end thereof and disposed at an offaxis angle from said longitudinal axis on a side opposite from said plurality of electron guns and operative to receive an electron beam returned from said target surface along a second path of opposite angular disposition with respect to said longitudinal axis; and

means for providing a magnetic field across said tube axis and oriented with respect to said axis and said first and second angular paths to divert said electron beam at said point from each of said first angular paths onto said longitudinal axis and to oppositely divert said returned electron beam at said point from said longitudinal axis onto said second angular path.

9. An electron beam tube according to claim 8 including:

a plurality of detectors each disposed off-axis from said longitudinal axis on a side opposite from said electron guns at a different angular extent and each operative to receive a respective electron beam returned from said target surface in response to a respective electron beam from each of said plurality of electron guns;

said means for providing a magnetic field being operative to divert each returned beam along a different angular path for receipt by respective ones of said detectors in accordance with the energy level of each returned beam.

10. A beam tube comprising:

an elongated envelope having a longitudinal axis;

a target surface supported within said envelope near one end thereof for receiving a beam of charged particles scanned thereacross;

focusing means supported within said envelope and disposed symmetrically around said longitudinal axis for focusing a beam of charged particles onto said target surface;

deflection means supported within said envelope between said focusing means and said target for scanning said beam across said target surface;

a charged particle source supported within said envelope near the other end thereof and disposed offaxis from said longitudinal axis and operative to direct a beam of charged particles along a first path angularly incident with said longitudinal axis at a point thereon intermediate said source and said focusing means;

a detector supported within said envelope near said other end thereof and disposed off-axis from said longitudinal axis at an equal and opposite angle from said source and operative to receive a beam of charged particles returned from said target surface along a second path of opposite angular disposition with respect to said tube axis as said first path; and

means for providing a magnetic field across said longitudinal axis and oriented with respect to said longitudinal axis and said first and second angular paths to divert said beam at said point from said first angular path onto said longitudinal axis and to oppositely divert said returned beam at said point from said longitudinal axis onto said second angular path. 

1. An electron beam tube comprising: an elongated envelope having a longitudinal axis; a target surface supported near one end of said envelope for receiving an electron beam scanned thereacross; focusing means supported within said envelope and disposed symmetrically around said longitudinal axis for focusing an electron beam onto said target surface; deflection means supported within said envelope between said focusing means and said target for scanning said electron beam across said target surface; an electron gun supported within said envelope near the other end thereof and disposed off-axis from said longitudinal axis and operative to direct an electron beam along a first path angularly incident with said longitudinal axis at a point thereon intermediate said electron gun and said deflection means; an electron detector supported within said envelope near said other end thereof and disposed off-axis from said longitudinal axis at an equal and opposite angle from said electron gun and operative to receive an electron beam returned from said target surface along a second path of opposite angular disposition with respect to said tube axis as said first path; and means for providing a magnetic field across said longitudinal axis and oriented with respect to said longitudinal axis and said first and second angular paths to divert said electron beam at said point from said first angular path onto said longitudinal axis and to oppositely divert said returned electron beam at said point from said longitudinal axis onto said second angular path.
 2. An electron beam tube according to claim 1 wherein said means for providing a magnetic field is operative in conjunction with the energy levels of said electron beam and said returned electron beam to produce equal and opposite angular diversion of said electron beam and said returned electron beam.
 3. An electron beam tube according to claim 1 wherein said first and second paths are in equal angular disposition on respective opposite sides of said longitudinal axis; and said means for providing a magnetic field is operative to produce equal and opposite angular diversion of the respective electron beams directed along said respective first and second paths.
 4. An electron beam tube according to claim 1 wherein said magnetic field is orthogonal to said longitudinal axis and said first and second paths are in a plane orthogonal to said magnetic field.
 5. An electron beam tube according to claim 4 wherein said magnetic field providing means includes an electromagnet having a gap oriented with respect to said longitudinal axis and said first and second paths and across which said magnetic field is produced.
 6. An electron beam tube according to claim 5 wherein said electromagnet includes a toroidal core having an energizing coil therearound.
 7. An electron beam tube according to claim 1 including: a second electron gun supported within said envelope near said other end thereof and disposed off-axis from said longitudinal axis and operative to direct a respective electron beam along a third path angularly incident with said longitudinal axis at said point and in a plane angularly rotated from the plane of said first path; and a second electron detector supported within said envelope near said other end thereof and disposed off-axis from said longitudinal axis on a side opposite from said second electron gun and operative to receive a respective electron beam returned from said target surface along a fourth path of opposite angular disposition with respect to said longitudinal axis as said third path and in the same plane thereof; said means for providing a magnetic field being also operative to divert said electron beam from said third path at said point onto said longitudinal axis and to oppositely divert said respective returned electron beam at said point from said longitudinal axis onto said fourth path.
 8. An electron beam tube comprising: an elongated envelope having a longitudinal axis; a target surface supported within said envelope near one end thereof for receiving an electron beam scanned thereacross; focusing means supported within said envelope and disposed symmetrically around said longitudinal axis for focusing an electron beam onto said target surface; deflection means supported within said envelope between said focusing means and said target for scanning said electroN beam across said target surface; a plurality of electron guns supported within said envelope near the other end thereof each disposed off-axis from said longitudinal axis by a different angle and each operative to direct a respective electron beam of different energy level along a respective first path angularly incident with said longitudinal axis at a point thereon intermediate said electron guns and said deflection means; an electron detector supported within said envelope near said other end thereof and disposed at an off-axis angle from said longitudinal axis on a side opposite from said plurality of electron guns and operative to receive an electron beam returned from said target surface along a second path of opposite angular disposition with respect to said longitudinal axis; and means for providing a magnetic field across said tube axis and oriented with respect to said axis and said first and second angular paths to divert said electron beam at said point from each of said first angular paths onto said longitudinal axis and to oppositely divert said returned electron beam at said point from said longitudinal axis onto said second angular path.
 9. An electron beam tube according to claim 8 including: a plurality of detectors each disposed off-axis from said longitudinal axis on a side opposite from said electron guns at a different angular extent and each operative to receive a respective electron beam returned from said target surface in response to a respective electron beam from each of said plurality of electron guns; said means for providing a magnetic field being operative to divert each returned beam along a different angular path for receipt by respective ones of said detectors in accordance with the energy level of each returned beam.
 10. A beam tube comprising: an elongated envelope having a longitudinal axis; a target surface supported within said envelope near one end thereof for receiving a beam of charged particles scanned thereacross; focusing means supported within said envelope and disposed symmetrically around said longitudinal axis for focusing a beam of charged particles onto said target surface; deflection means supported within said envelope between said focusing means and said target for scanning said beam across said target surface; a charged particle source supported within said envelope near the other end thereof and disposed off-axis from said longitudinal axis and operative to direct a beam of charged particles along a first path angularly incident with said longitudinal axis at a point thereon intermediate said source and said focusing means; a detector supported within said envelope near said other end thereof and disposed off-axis from said longitudinal axis at an equal and opposite angle from said source and operative to receive a beam of charged particles returned from said target surface along a second path of opposite angular disposition with respect to said tube axis as said first path; and means for providing a magnetic field across said longitudinal axis and oriented with respect to said longitudinal axis and said first and second angular paths to divert said beam at said point from said first angular path onto said longitudinal axis and to oppositely divert said returned beam at said point from said longitudinal axis onto said second angular path. 